Deployment of distributed power generation such as wind and solar power continues to grow. Most of these power systems rely on numerous switched-mode power processors (SMPPs) which facilitate connection of a power station to a power system grid. SMPPs are fully described in Mohan, et al., Power Electronics: Converters, Applications, and Design, (3d ed., 2002; hereinafter, “Mohan (2002),” which is incorporated herein by reference. SMPPs can produce undesirable harmonic distortion into the power system. The standard for power system harmonic content is guided by IEEE 519 1992 (hereinafter, “IEEE 519”), which for example, considers a single point of interconnection as the measuring point for signal distortion on the power system. More importantly, IEEE 519 primarily evaluates signal distortion as harmonic multiples only of the fundamental frequency on AC power systems. But, more importantly, IEEE 519 admits that “the effects of harmonics on electric equipment, appliances, etc., are not fully understood,” and goes on to recommend a probabilistic method of evaluating harmonic distortion, such that if limits are only exceeded for a short period of time, the condition could be considered acceptable. Despite adherence to the aforesaid standard, there is evidence that multiple SMPPs can collectively construct undesirable and potentially damaging harmonic distortion that may compromise the integrity of the power grid.
Various methods have been employed for reduction of harmonic content on polyphase power systems, however all of them are limited in that they eliminate only a particular class of harmonics (namely, integral multiples of the AC fundamental), and in that each is capable of being applied only at a single node of a power system. For example, U.S. Pat. No. 6,510,063, incorporated herein by reference, teaches the use of pulse wide modulation (PWM) for elimination of harmonics, but only at a given node at the interconnection of the power conversion unit.
The fact that the prior art applies harmonic mitigation only at a local, single point, is a particularly egregious limitation in the case of wind power. U.S. Pat. No. 5,798,631, for example, incorporated herein by reference, teaches a variable-speed, constant-frequency (VSCF) system utilizing a doubly fed induction machine to provide for harmonics and reactive compensation. Control of harmonic content in the '631 patent, however, is provided at the mains of a wind turbine generator or, more specifically, at a converter interfacing between the generator and an output to the grid. The '631 Patent, however, leaves open the questions: How is the signal to be modified at a remote location, and how can signal quality be improved at the remote location when many SMPPs or converters feed into the system?
Similarly, while it already known how to determine the impact of non-linear load harmonics on voltage distortion at a point of common coupling, such techniques are limited to singular points within a system, and do not characterize the system in its entirety. Thus, for example, dynamic harmonic filters are known in the context of AC power systems, however they only monitor, and improve performance at, specific single points on the system.
Currently practiced techniques are prone to damage both utility and customer equipment, as suggested, in the context of transformers in Parthemore, et al., “Gassing in Wind-Farm Transformers,” Windpower Eng. & Dev., pp. 49-51, (April, 2012), incorporated herein by reference. It would be extremely advantageous for a method to provided that would cancel, or attenuate, undesirable signals before their passage through a susceptible component of a power transmission system and before extensive damage is done.
Currently, waveform distortion causes premature aging of electrical equipment, excessive heating, current and voltage stresses that cause failure in electrical components, loss of real power production and consequently loss of revenue. On polyphase AC power systems and DC power systems, distortion of the desired signal has consequences. In the past, power systems engineers called this “harmonic distortion.” The problem is larger however, and encompasses all manner of signal distortion.
When new power plants or new components are placed onto a power system, a harmonic study is required to discover or predict harmonic issues in the power system that operators and owners may find unacceptable. A plant developer may sometimes be required to provide a harmonic study for the electrical distribution system. The intent of such a harmonic study is to confirm that the specified and supplied equipment will operate properly when installed as specified in the system and will not adversely impact the operation of other equipment.
Currently, harmonic studies include various portions of the electrical distribution system, from the normal and alternate sources of power down to each load shown on the “one-line diagram” used to depict the system. However, some power system elements within the power system are summarized and not delineated as well as others. The concept of using harmonics is applied not only to operation during normal conditions, but also to alternate operational configurations, emergency power conditions and any other operations which could result in harmonic distortion exceeding prescribed standards.
In accordance with prior art practice, harmonic analysis is typically performed with a simulation which is a series of harmonic calculations, and discusses some of the results at the following locations:                1. The point of common coupling        2. Primary side of each unit substation (normal power)        3. The bus of each switchboard (normal and alternate power)        4. Each alternate power source (including generators)        5. The collector system busses within the a distributed generation system        6. Various Loads within the power system        
The resultant harmonic analysis includes recommendations for mitigating the total harmonic voltage distortion or total current demand distortion on the system if the combination of loads exceeds or violates the limits of various standards. Also, the analysis includes recommendations for mitigating the impact of the harmonic distortion on plant equipment or processes if the levels are such that equipment or processes may be impaired.
However, harmonic analysis is deficient in modern power systems and, even though switch mode power supplies and other modern piecewise or nonlinear elements are mentioned in IEEE 519, the standard is brief and insufficient on characterizing their behavior on the power system. These newer topologies, such as switch mode power processors, produce distortion that is not a whole number multiple of the fundamental frequency on the power system
Another deficiency in the current state of the art and existing processes is that, typically, signal distortion is handled only in the planning phases of a power system that is to be modified or a new power system. For example, if a new power plant is to be installed, the harmonic study would be performed to assess it impact on the power system with respect to power quality. This process uses models of the power system to predict what the harmonic impact will be and does not assess the actual or measured impact. However, some plants are required to measure and validate their models after the plant or device is installed.
Yet another deficiency in the state of the art of harmonic distortion is that harmonic distortion is predicated on whole number multiples of the fundamental frequency on the power system. This infers that the cause of signal distortion has a single mode or cause, and lumps signal distortion into one elemental category and does not break out all the causes of distortion on the power system. However, new switch mode power supplies placed either on the load or the sources do not always distort the signal at a whole number multiple of the fundamental frequency. Consequently, current state of the art of harmonic analysis is incomplete when attempting to consider all the causes of signal distortion.
For example, in the ideal case for three phase power systems, including wind turbines and wind parks, all the waveforms would be considered sinusoidal with no distortion occurring at a fundamental frequency such as 60 Hz in the Americas and 50 Hz in Europe and elsewhere. This includes the waveforms of all currents and voltages. However, in reality this is not the case. The waveforms are distorted. Most electrical equipment will create distortion in the waveforms. The signal distortion can be problematic because it may damage, age, or reduce efficiency of equipment within the power system. In the past, before switch mode power processors became popular to use on the power system, engineers knew what type of signal distortion was going to occur and utilized tried and true methods to remedy problems that may arise. However with new distributed generation power plants such as wind farms, which use switch mode power processors, plant operators have found old methods do not work as well as they did in the past. For example, the introduction of large scale wind generation on the North American and European power systems which use large scale switch mode power processors (of greater than 100 kW) signal distortion is observed on power systems, and the signal distortion is not a whole number multiple of the fundamental (e.g., 50 Hz, 60 Hz).
Generally, signal distortion is represented in the frequency domain by breaking down the time based signal in to a frequency-based equivalent at frequencies that are based upon whole-number multiples of the fundamental. Such representation may be referred to as harmonic analysis. The output of this analysis creates a bar graph spaced equally showing magnitudes of the signal amplitudes at multiples of the fundamental. Each bar is called a harmonic of the fundamental. For example, if the bar is the 11th bar in the graph and the fundamental is 60 Hz, this bar would represent the 11th harmonic at given amplitude, its frequency being 11 multiplied by 60 Hz, or 660 Hz.
For example, in order for a power processor to create or synthesize a signal on the power system at the fundamental frequency such as 60 Hz, it uses techniques, including but not limited to, pulse width modulation (PWM), techniques, and rotational transformations, to approximate the desired current and/or voltage waveforms at its terminals. However, in order to create these desired waveforms, the algorithms and PWM switching equipment will take an ideal waveform signal and distort it. Furthermore, PWM switching equipment and algorithm will replicate an already distorted waveform on the power system. The spectrum of the distortion including the phase and magnitude vary as the frequency increases. It has been observed that the magnitude of the distortion decreases as the frequency increases, but this is not always the case. In distributed generation and in distributed loads which use switch mode power supplies or power processors the distortion can add up at common location on the power system and the resulting distortion at higher frequencies may become unacceptable.
To cite one example of waveform distortion, modern wind turbines use switch mode power processors (SMPPs) to improve performance and control torque. SMPPs, in broad terms, attempt to process and control the power by supplying either/or voltages and currents in a form that best suits requirements. However SMPPs can only approximate the ideal wave form for the load, often by replicating a template wave-form sampled from the grid to which it is connected. Consequently, these imperfect wave forms created by SMPPs contribute to the overall distortion. Also, SMPPs are designed from the view point of supplying loads on an individual basis; with the exception of SMPPs independently interfering with each other in the hopes of creating less distortion. Presently, the signal distortion caused can be characterized on an individual basis but not controlled on an aggregate basis at the point of common coupling or within the entire power system. Furthermore, standards used for testing of power system quality rely on the test of a single node, where single or multiple devices such as SMPPs, perhaps hundreds, will comprise a single power plant, such as in a wind powered plant.
Currently, SMPPs are not coordinated with each other to minimize harmonic content and or signal distortion. Typically, they operate individually without coordinating with each other or any other power system component. However, on an individual basis they attempt to minimize their harmonic content. Generally, manufactures publish that their SMPP meet acceptable standards such as IEEE 519. Ironically, some manufactures would argue that the aggregate harmonic content for several SMPPs is less since they would interfere with each other. However, the converse is equally true since they are not coordinated and may constructively increase the level of distortion. It has been observed that the harmonic content or signal distortion would cyclically increase at different times and various amplitudes on the power system. It has also been observed in areas where there is large distributed generation. Basically, the level of harmonic content at a specific point within the power system is random and may or may not exceed acceptable standards. Currently, there exists no mechanism to know when or where the distortion may add up beyond acceptable standards since there are no standard measurements analyses that would capture these phenomena in their entirety.
As another example, before the popular implementation of power semiconductors, the main sources of waveform distortion were electrical arc furnaces, the accumulated effect of fluorescent lamps, and to a lesser extent electrical machines and transformers. The increasing use of power electronic devices in the wind turbine industry for the control of power apparatus and systems has been the reason for the greater concern about waveform distortion and its effect on wind turbines and related power system components in recent times.
A SMPP can be viewed as a matrix of static switches that provides a flexible interconnection between input and output nodes of the electrical power system of a wind turbine generator, or more generally, electrical equipment on the power system For example, through these switches, power can be bidirectionally transferred to the generator of the wind turbine. Because of their considerable power ratings, three-phase SMPPs may be the main contributors to the distortion problem. For clarification, the SMPP can perform both functions of rectification and inversion and are used for powers transfers from AC to DC or DC to AC, respectively, and the term “conversion” may be used when the power electronic device has bi-directional power transfer capability. According to the relative position of the firing instant of the switches, whether cycle to cycle or subcycle, four different power electronic control principles are in common use: (1) Constant phase-angle control produces consecutive switch firings equally spaced with reference to the irrespective commutating voltages. (2) Equidistant firing control produces consecutive firings at equal intervals of the supply frequency. (3) Modulated phase-angle control produces time-varying phase-modulated firings. (4) Integral cycle control selects an integer number of complete cycles or half cycles of the supply frequency. And one additional and uncommon use is flux-path switching, where the flux path is switched and modulated through coils.
Signal distortion and harmonics on the power system can damage wind turbine generators in a multitude of ways. As one example, the problem of over voltage saturation is particularly damaging to transformers and wind turbine generators. In the case of transformers connected to a converter following load rejection and depending how far out on a radial collector circuit the WTG is located, it has been shown that the voltage at the converter terminals can exceed 1.43 per unit, thus driving the converter transformer deep into saturation. The symmetrical magnetizing current associated with wind turbine transformer core saturation contains odd harmonics. If the fundamental component is ignored, and if it is assumed that all triple harmonics are absorbed in delta windings, then the harmonics being generated are of orders 5, 7, 11, 13, 17, 19 . . . , i.e., those of orders 6k±1, where k is an integer. When considering a wind turbine with a wound rotor generator it easy to show how the stator generator terminals experience this. An important note is that for a doubly-fed wound rotor generator (DFIG), the power flow is bi-directional depending on the rotor RPM. For the induction generator the fifth and seventh harmonic combine to produce a pulsation or hammer torque that can damage the wind turbine drive train. Other higher order harmonics that combine in a similar nature will also produce damage to the drive train.
The current state of harmonic analysis considers the harmonics at a particular point or line at a particular time or possibly a series of snapshots such as the point of common coupling (PCC). The issue with this is that the picture is static and not fluid. The existing methods of analyses do not include traveling waves on the power system; nor do they consider the signal propagation delays through plants and controllers. One salient limitation of contemporary harmonic analysis is that it does not consider traveling waves and signal propagation delay(s) within the control system or distributed control system. A distributed control system is made up of many controls working separately or together or some combination of the two within the wind plant or park. One problem resulting from this is that an engineer does not have an idea of what is causing the harmonic or, better stated, signal distortion, since we know harmonic analysis is limited in the first place, and unless it is a usual cause that can be easily identified, the engineer is unable to arrive at a determinate solution.
An analogy to rogue waves on the ocean are waves that usually occur as one gigantic wave passing a given point. For power systems with SMPPs traveling waves are not usually a concern, however wind plants or, more generally, distributed generation with large SMPPs are new and this phenomena is only now being revealed. Most likely it will show up on other power systems with large switch mode power supplies as well. For higher order harmonics above the 25th and especially beyond the 50th, these waves can propagate on the collector and local transmission and distribution (T&D) systems. Sometimes they add up to increase in amplitude and create an impulse on the power system that can be damaging to equipment. Currently, SMPP controllers are not coordinated to handle this issue.
Another type of rogue wave is an indirect one that is caused in part by the control system as well as by the power system. The control system issues a response and that response flows out on the power system. From there another controller sees the response and creates its own response sending it out on the power system. Several controllers can interact like this, sympathetically and in phase. Each controller on its own will have low signal distortion, however, since they are in phase at a particular node, a signal distortion may increase or be compounded. The aggregate effect is that the magnitude of a given frequency has increased and is damaging equipment. However, the aligning of the distributed control systems is transitory in nature and constructively aligns or destructively interferes, thus hiding itself from engineers and technicians, and reoccurring when conditions enable rogue wave construction.
For example, on large wind plants and within the power system in general, these waves start with the flow of charge, called current, and/or accelerated flow of charge (di/dt) on the power system at the wind plant. The distortion of the current waveform coupled with the impedance produces voltage harmonics. This signal distortion of the voltage is added to the existing distortion on the transmission system. Both combine to create the signal observed on the power system. Due to the complex nature of the power system, collector system, and distributed generator (DG) controls, it is difficult to identify how and when the distortion problems will construct or occur. What is needed are devices that allow engineers and technicians to change the system characteristics as needed to prevent power system signal distortion/harmonics from damaging equipment.
Because of the deficiencies in existing technology, some of which have been enumerated and described above, it is desirable to improve efficiency, power transfer reliability, and longevity of equipment, and to those ends the invention described below is directed.